Drill pipes are threaded together in use in drill strings during drilling, for example of oil or gas wells. One end of a drill pipe has an internally threaded box or female connection, while the other end has an externally threaded pin or male connection so that the drill pipes may be threaded together. The drill pipe adjacent the box or pin is enlarged to form an upset and provide strength for the joint, but to reduce the cost of making the drill pipe, the upset typically does not extend very far down the pipe length, typically not much more than about twice the pipe diameter. The internal bores and threads of the box and pin are machined precisely to allow the box and pin to seal together on sealing surfaces at either end of the threads, and also perhaps at intermediate points between threaded portions.
During drilling, stresses on the drill pipe and especially on the boxes and pins cause the interior surface of the box and the exterior surface of the pin to become worn, such that they do not seal together, and fluid within the drill pipe can leak. Since drill pipe is expensive, rather than throw out the drill pipe when it is worn, drilling contractors will repair or have the drill pipe repaired by refinishing the boxes and pins. The refinishing is carried out by machining a new set of threads and sealing surfaces deeper into the box and pin respectively. This eliminates a part of the enlarged portion of the box and pin. After several repairs, the enlarged portion of the box or pin is gradually eliminated until no further repairs can be carried out and the drill pipe is discarded.
The machining of the drill pipe is typically carried out using a lathe having a chuck with jaws mounted in a plane at one end of the lathe. The chucks hold and rotate one end of a drill pipe against a working tool. The other end of the drill pipe is held in a steady rest. Often, the drill pipe will be bent at the end, with the consequence that when cradled in the lathe, the portion of the end being worked on does not rotate precisely in a circle. In this instance, a large amount of metal on the end of the drill pipe may need to be taken off in order to obtain a precisely configured box or pin. This limits the number of times the drill pipe can be repaired, and consequently increases the operating costs of the drilling contractor.